At a location such as the offices of a business, there typically are many telephones that may be used to communicate among themselves as well as over the public switched telephone network (PSTN) with other telephones at other locations. In such an application, it is typical practice to provide a PBX at the office location for facilitating the connections among the telephones at the office location.
A basic PBX installation consists of a cabinet to which a number of phones may be connected, and which contains the circuitry for interconnecting those phones. The number of phones that may be serviced by one cabinet is limited by the amount of circuitry that may be accommodated within the cabinet. A typical number of phones serviced by one PBX cabinet is on the order of 50 to 100.
If it is desired to service a number of phones greater than that which may be serviced by one PBX cabinet, the PBX may be augmented by additional PBX cabinets connected to the original one. FIG. 1 depicts two PBX cabinets 101, each servicing a number of phones. The two PBX cabinets 101 are connected by a wire path 102, through which a phone connected to one of the PBX cabinets may communicate with a phone connected to another of the PBX cabinets path 102 may be other forms of hard path other than a wire path, such as a fiber optic path. All the cabinets comprising a single PBX are in the same general location, typically no more than several hundred feet apart. By using repeaters, the distance between cabinets can be extended to several miles.
As the distance between PBX cabinets increases, (e.g., when interconnecting phones in different buildings on an office campus) it may be convenient to interconnect the PBX cabinets by means other than a copper or fiber optic path. For example, it may be convenient to interconnect PBX cabinets by an Internet Protocol (IP) connection. An IP network may already be in existence. However, when interconnecting PBX cabinets it is necessary to transmit not only data but also a timing reference. Copper path 102 (FIG. 1) carries a timing reference. However, an IP connection (e.g., an Ethernet connection) transmits data asynchronously, i.e., without a timing reference. Also, an IP network routes each packet according to current network conditions, and thus as conditions change packets may take different routes and may be held up for varying amounts of time. When an IP network is lightly loaded and functioning under optimal conditions, it probably delivers output at substantially the same rate as its input, but under adverse conditions, such as heavy load, packets may be delayed by markedly different amounts of time, and may even be lost entirely. Packets may thus arrive at the destination at a different instantaneous rate and in a different order than that in which they were transmitted, may arrive too late to be acted upon, or may not arrive at all.
There is thus a need to reconstruct a timing reference for synchronous data after transmission thereof by an asynchronous, unreliable communication medium which may deliver data out of order, which may deliver data at an instantaneous rate substantially different than its transmission rate, and which may lose portions of the data.
It is an advantage of the present invention that a system and method is provided for reconstructing a timing reference for synchronous data that has been transmitted by an asynchronous communication medium.
It is a further advantage of the present invention that a system and method is provided for reconstructing such a timing reference for synchronous data that has been transmitted out of order and with data loss by an unreliable asynchronous communication medium.
These and other advantages of the invention will become apparent to those skilled in the art from the following description thereof.